Actuating devices for the alternating coupling of the particular clutch of a drive shaft to a transmission input shaft are already known from DE 34 46 460 C2, one clutch including a disc spring being disengaged in each case by one control mechanism including an annular piston coaxial to the clutch axis and connected to the relevant disc spring. The two annular pistons are radially offset from one another in the housing.
Another possibility for engaging a drive shaft with a transmission input shaft via alternate coupling of the particular clutch of a double clutch and thereby improving the hysteresis properties of the aforementioned device is described in DE 199 53 091 C1. The hydraulic surface of the two annular pistons lying coaxial to the clutch axis, as described in DE 34 46 460 C2, is in this case replaced by a plurality of subsurfaces, which are derived from the pressure surfaces of rod-shaped single pistons. When actuated, pressure fluid is applied to these single pistons, referred to here as servomotors. Three servomotors having their axes positioned parallel to the clutch axis are assigned to each clutch. All 6 servomotors lie on a graduated circle coaxial to the clutch axis, the 3 control pistons assigned to a clutch being in each case positioned 120° in relation to each other and pressure being constantly applied synchronously to them in the operating state.
However, a disadvantage of these devices is that the application of pressure to a piston or to a piston system (made up of three pistons) is never centrical for each individual piston so that pressure is only applied to each eccentrically. This results in the pistons being tilted in relation to their bearing surface in the cylinder, causing increased friction and finally wear of the pistons and their bearing surfaces.
Furthermore, the length of the pistons is adapted to the corresponding bearing surface and accordingly to the length of the cylinders, which has a negative influence on the weight of the engaging/disengaging device.
Moreover, the three pistons in DE 199 53 091 C1 are not mechanically connected to one another so that in the case of pressure forces of varying strength, each piston is extended a varying distance. This results in the bearing tilting and thus colliding with the housing. Another disadvantage is the direct connection of the three pistons with the particular bearing. This causes a concentrated loading of the bearing and thus reduces the life of the bearing.